1,8-Dihydroxy-9-anthrone, or anthralin (dithranol), has been used since 1916 for the treatment of psoriasis and certain other skin diseases. There are, however, several adverse effects involved in the use of anthralin. First, anthralin has a strong inflammatory effect on the skin. Furthermore, it stains skin, hair and clothes a strong purplish brown colour, a factor which limits its use esthetically. In order to eliminate these adverse effects, certain derivatives have been prepared from anthralin, e.g. 1,8,9-triacetoxy-anthracene (Exolan.RTM.). Exolan.RTM. does not stain or inflame the skin to the same degree as anthralin. Its anti-psoriatic action is, however, to such a considerable degree lower that it has not been used extensively in medicine. ##STR1##